second of five main divisions (in ascending order) in the Tertiary System, representing all those rocks on a global basis that were deposited during the Eocene Epoch (57.8–36.6 million years ago). It designates a subdivision proposed in 1833 by the Scottish geologist Charles Lyell based on the percentage of fossil mollusks in Eocene strata with living representatives. Foraminifera, calcareous nannofossils, and radiolaria biozones are most widely used for biostratigraphic zonation within the series. The base of the Eocene Series has yet to be defined by a global stratotype section and point (GSSP), but the top of the series is well-defined by the base of the overlying Oligocene Series, as designated by its GSSP.